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The formation of microscale single- and double-emulsion droplets with various sizes is crucial for a variety of industrial applications. In this paper, we report a new microfluidic device which can actively fine-tune the size of single- and double-emulsion droplets in liquids by utilizing controllable moving-wall structures. Moreover, various sizes of external and internal droplets for double emulsions are also successfully formed by using this device. Three pneumatic side chambers are placed at a T-junction and flow-focusing channels to construct the controllable moving-wall structures. When compressed air is applied to the pneumatic side chambers, the controllable moving-wall structures are activated, thus physically changing the width of the microchannels. The size of the internal droplets at the intersection of the T-junction channel is then fine-tuned due to the increase in the shear force. Then, the internal droplets are focused into a narrow stream hydrodynamically and finally chopped into double-emulsion droplets using another pair of moving-wall structures downstream. For single emulsions, oil-in-water droplets can be actively fine-tuned from 50.07 to 21.80 under applied air pressures from 10 to 25 psi with a variation of less than 3.53%. For a water-in-oil single emulsion, droplets range from 50.32 to 14.76 with a variation of less than 4.62% under the same applied air pressures. For double emulsions, the sizes of the external and internal droplets can be fine-tuned with external/internal droplet diameter ratios ranging from 1.69 to 2.75. The development of this microfluidic device is promising for a variety of applications in the pharmaceutical, cosmetics, and food industries.